According to Microsoft, a zero day flaw in Internet Explorer (IE) , which impacts all versions of the browser, is being actively exploited in the wild. Reports of exploitation, according to Microsoft, seem to have criminals focused on IE versions 8 and 9.

Complicating matters, researchers at Websense have discovered that nearly 70 percent of Windows business users are susceptible to this IE zero-day exploit.

"We reviewed third-party telemetry feeds from real-time global internet requests to determine the initial scope. While the exploit appears to affect all versions of IE, at the moment attacks only seem to be targeting users of IE8 and IE9 who are running Windows 7 and XP operating systems," Websense's director of security research, Alex Watson, said in a statement.

According to the advisory, Microsoft said that IE installations running on Server 2003, 2008 and 2012 will mitigate the vulnerability due to its installation parameters. Experts have suggested that the scope of the problem is bad enough that Microsoft will likely go out-of-band to release a fix.

"This recently discovered Internet Explorer zero day vulnerability is bad. Users and administrators should take immediate action to mitigate the risk. Considering the timing, I personally expect to see an out of band patch from Microsoft," Rapid7's senior manager of security engineering, Ross Barrett, told CSO.

"All versions of IE are affected, which means that this vulnerability has likely been present since IE 6 was released in 2001. The fact that it is getting attention now is due to either a noticeable volume [of attacks] or impact of active exploitation in the wild. It may have just been discovered last week, or it may have been in the private toolkit of the world's best malware writers for more than a decade. This is as severe as any browser issue can be."

Patrick Thomas, security consultant at Neohapsis, said in a statement to CSO that he expects to see this vulnerability added to exploit kits and see a wide general use by criminals within weeks.

"So while this exploit was initially constrained to a small group of targets, it will likely be included in various commercial exploit kits and in wide, general use within the next 1-5 weeks."

The discovery of this latest Internet Explorer flaw is further proof that attackers continue to target low-hanging fruit, Thomas explained. The use of Address Space Layout Randomization (ASLR) is one of several defensive technologies baked into modern programs and libraries. Such protective measures make attacks like the ones observed targeting Internet Explorer significantly harder.

"Its no coincidence that attackers are targeting a dynamic-link library (DLL) that did not get compiled with ASLR. Enterprise administrators should be aware of what software on their networks uses and does not use modern, built-in protections, including DEP, ASLR, and stack protections, and consider upgrade plans or increased patching priorities to mitigate risks from these more easily-targeted programs," Thomas said.

Microsoft is encouraging IT administrators to install the FixIt patch (CVE-2013-3893 MSHTML Shim Workaround) to mitigate potential damage, until a proper fix is issued. The FixIt only applied to 32-bit installations, so other mitigations such as EMET 3.0 and 4.0 are also encouraged.

Unless Microsoft issues an out-of-band patch, the next round of security patches from Redmond are due October 8.